Method and means for growing and treating crystals



April 1963 A. J. MARINO, JR., Em. 3,086,850

METHOD AND MEANS FOR GROWING AND TREATING CRYSTALS 2 Sheets-Sheet 1 Filed June 17, 1959 SOURCE A ND TUNER INVENTORS.

A/vr/m/w J. MARIA/0, JR, BY DONALD c. SEELEV A ril 23, 1963 A. J. MARINO, JR.. ETAL METHOD AND MEANS FOR GROWING AND TREATING CRYSTALS 2 Sheets-Sheet 2 Filed June 17, 1959 INVENTORS ANTHONY J MAR/N0, JR, DOA/Alb C 565(5) BY AGENT METHOD AND MEANS FOR GROWING AND TREATING CRYSTALS Anthony J. Marino, Jr., Teaneck, and Donald C. Seeley, Verona, N.J., assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed June 17, 1959, Ser. No. 820,938 7 Claims. (Cl. 23-273) This invention relates to a method and means for growing crystals and more particularly to a method and means for growing crystals by fusion.

With the advent of transistors and other semiconductor materials which exhibit solid-state phenomena, various schemes for obtaining these semiconductor materials in single crystal or polycrystalline form have come into being. Pulling techniques, floating zone methods and gaseous decomposition schemes are among the more wellknown methods which provide semiconductive materials in either single crystal or polycrystalline form. These prior art schemes often leave much to be desired in that no single method is universally applicable for the production of all types of semiconductive materials. Techniques, however, have been introduced which, by the utilization of powdered semiconductive materials, a flame, and a combustion supporting atmosphere have expanded the growing of semiconductive crystals to include materials such as ferrites. As has been mentioned, however, none of these schemes is universally applicable in that any one technique can grow crystals of any type semiconductor. It can be seen, therefore, that a technique which is capable of growing crystals of germanium and silicon as well as single crystals of ferrites and other high purity materials such as metallic elements and dielectric would have wide application and would tend to supplant present crystal growing techniques.

It is, therefore, an object of this invention to provide an improved method and means for growing crystals.

It is another object of this invention to provide crystal growing methods and means which are applicable not only to the growth of crystals of semiconductive materials but also to the growth of materials which are conductive, such as the elemental metals.

It is a further object of this invention to provide crystal growing methods and means which are applicable to the crystal growth of materials which are not ordinarily conductive but which become conductive upon heating.

Another object of this invention is to provide crystal growing methods and means 'which are applicable to the growth of crystals of dielectric materials.

It is a still further object to provide crystal growing methods and means which are continuous in operation.

Yet another object of this invention is to provide methods and means for growing crystals wherein impurities maybe introduced in solid or vapor form or wherein the constituents of the feed may be varied without interrupting the process.

One of the features of this invention is the method of growing crystals of a given material which is conductive at a temperature below its melting point by subjecting a seed of the given material to induction heating to render a portion of the seed molten while the remainder of the seed is in solid state, thereby inducing eddy currents in the molten portion which stir the molten portion and by dissolving additional material in said molten portion and withdrawing the seed from the zone of induction heating to control growth of the solid portion thereof.

Still another feature of this invention is the utilization of doping materials in particle or vapor form to change the properties of the crystal being grown.

The above-mentioned and other features and objects tates atent of this invention and the manner of attaining them 'will become more apparent by reference to the description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a view in elevation with parts in section of apparatus according to the principles of this invention; and

FIG. 2 is an enlarged cross-sectional view of the induction coils, field concentrator and molten tip of the crystal seed.

Referring now to FIG. 1 there is shown a partial crosssectional view of the apparatus utilized in the practice of this invention. A seed 1 of the highest purity obtainable of the material desired to be grown is mounted on and fastened to a pedestal member 2. Pedestal 2 is in turn mounted on and fixed to rotatable shaft 3. Shaft 3 is rotated by the action of motor 4 and beveled gears 5, one of which is coupled to shaft 3. Shaft 3 is held by bearings 6, the outer races of which are coupled to a rack 7. Rack 7 is capable of motion vertically upwards and downwards by the engagement of the teeth of the rack 8 with a gear 9 which is actuated by a handle or other driving member 10-. Thus, shaft 3 is held by bearings 6 in fixed position relative to rack 7, but is rotatable about its own axis and is capable of being moved vertically by virtue of the combination of rack 7 and gear 9. Two serially connected radio frequency coils 11 and 12, an annealing coil and a work coil, respectively, are shown disposed in juxtaposition to high purity seed 1. Coils 11 and 12 are actuated by a radio frequency source and tuner 13 which delivers radio frequency energy power to coils 11 and 12 in the form of radio frequency power. Coil 11 is disposed below coil 12 and is spaced from but coiled circumferentially about seed 1. Work coil 12 is disposed slightly above seed 1 and is disposed internally of field concentrator element 14. Field concentrator 14- is a hollow truncated cone made of copper and work coil 12 is substantially conformal with the inner surface of concentrator '14 such that it forms a helical coil tapering from a large diameter to a small diameter at a point nearest the seed. Field concentrator 14 is utilized to direct or concentrate the radio frequency field in the region or zone of the tip of seed 1 to cause the tip of seed 1 to become molten due to the action of the radio frequency field known as induction heating. Feld concentrator 14 has a gap 14' radially thereof to prevent the concentrator from acting as a shorted turn to prevent bum-out of the radio frequency source 13. Quartz tube 15 leads from one or more reservoirs 16 through work coil 12 to a point adjacent the tip of seed 1 which is to be made molten upon the application of radio frequency energy. Reservoir 16 is utilized to hold particles of the material of which a single crystal is to be made utilizing the technique of this invention. Quartz tube 15 is utilized merely as a convenience to guide the particles being dispensed from reservoir 16 to the region of the molten pool on the tip of seed 1. A tapper arrangement 17 driven by motor 18 is utilized to cause the particles in reservoir 16 to be agitated at a given rate and passed through mesh 19 to the molten tip of seed 1.

Upon application of radio frequency energy from source 13 to induction coils 11 and 12, seed 1 becomes molten at its tip. Seed 1 is then rotated by actuating motor 4 which, through bevel gears 5, actuates shaft 3 in a rotational manner. Motor 18 actuates tappet 17 and particles of feed material are dispensed through quartz tube 15 to the molten tip of seed 1. As the particles dissolve in the molten tip and accumulate therein, the rotation of the seed causes or permits a uniform build-up in the molten region. As the particles accumulate in the molten tip of seed 1, the seed 1 must be gradually withdrawn so that the distance d between the tip of seed 1 and the edge of field concentrator 14 is held at a substantially constant value. The withdrawal of seed 1 is accomplished by actuating handle which turns gear 9 thereby gradually withdrawing shaft 3 at whatever rate is desired depending upon the rate. of accumulation of particles in the. molten tip of seed 1. As seed 1 is withdrawn the annealing coil 11 maintainsthe region below the molten tip of seed 1 at a temperature just below the melting point. In this manner, sharp temperature gradients which lead to cracking of the crystal are eliminated. Coils 11 and 12 are shown serially connected but they may be connected in parallel without departing from the operation of this apparatus as just described. Thus, it can be seen that a crystal may be grown continuously. and that by using two or more hoppers, as shown, various materials may be introduced into the molten pool thereby doping the crystal being grown or otherwise varying some physical or chemical property of the crystal being grown.

An enclosure 20 of dielectric material, designed to withstand considerable internal and external pressure is disposed about induction coils 11, 112 and seed 1 and isolated from the ambient atmosphere by O-ring seals 21, 22 which are disposed about quartz tube 15 and rotating shaft 3, respectively. Input and output ports 23, 24, respectively, permit any of the various gases such as oxygen, hydrogen, argon or any mixture of gases desired to be introduced and removed from the region of seed 1 thereby providing the various atmospheres required in accordance with the teaching of this invention. The leads 25, 26 of induction coils 11, 12 from radio frequency source and tuner are fed through enclosure by means of insulators 27, 28. A vacuum sufficient for the needs of this invention is maintainable by virtue of seals 21 and 22 and considerable pressure may also be applied and maintained within enclosure 20 by these seals. In connection with the use of a vacuum or various gases, it should be noted that the design of heating coil 12 is varied in accordance with the type of atmosphere used. For instance, parameters such as pressure and ionization capability of the atmosphere being utilized determines the design of the coil 12.

Referring now to FIG. 2, there is shown therein an enlarged view of the area of the molten tip of the seed which includes a cross-sectional view of the seed 1, coils 11 and 12 and field concentrator 14. It is Significant to note that seed 1 has a convex shape. This shape is due partially to the surface tension of the molten tip of the seed, to levitation by the radio frequency field but is due principally to magnetic stirring by the radio frequency energy of the molten tip of seed 1. The interface between the molten tip of the seed and the seed which is in substantially solid form is shown by line aka. The shape ofthis interface is due to the manner in which the molten pool is agitated by magnetic stirring and thermal gradients within the tip of seed 1. The temperature at point b is the greatest and the molten material travels away from point 12 towards points a and 0, respectively, which are cooler, along the paths X and Y. As the material moves away from point b, material from points a and 0 moves toward point I) to take the place of the material flowing away from point b. In this manner, a circulation of molten material is set up which travels over path X and Y. This flow occurs in a radial direction from. point b towards the outer edge of seed 1 in the manner just described and, therefore, a section through any diametric portion of seed 1 would appear to have a flow as shown in the cross-sectional view of the present figure.

The field provided by work coil 12 and field concentrator 14 is shown in FIG. 2 as lines of force 29' perpendicular to the turns of coil 12 and entering into the molten tip of seed 1, as shown, to provide heating thereof so that the tip of seed 1 becomes molten. The coil 12 and concentrator 14 are so designed that the lower most turn near the tip of seed 1 is in contacting relationship with the portion of field concentrator adjacent it. In this manner, the field concentrator becomes an extension or the lowermost turn of coil 12 and by properly adjusting the length of the turns of coil 12, such that the currents in the coil are all in the same direction at a given instant, the field 20 is concentrated in the tip of seed 1. Quartz tube 15 is shown guiding particles of the material of which the crystal is to be formed toward the molten tip of seed 1. It should be noted at this point that the particles 30 enter the molten tip of seed 1 in solid form and dissolve therein and are not acted upon by the radio frequency field to cause melting of the particles 39* before particles 30 enter the molten tip of seed 1. Annealing coil 11, is shown coiled about and spaced from seed 1 and slightly below the molten tip of seed 1 to provide, by induction heating, sufficient thermal energy to the solid portion of seed 1. In this manner, a sutliciently high temperature is maintained by coil 11 to prevent cracking due to sharp thermal gradients.

Returning to FIG. 1, a resistive element 31 is shown coupled about seed holder 2. This element is utilized to apply radiant energy to materials which require initiation by thermal energy to make them conductive to radio frequency energy. These materials include silicon and barium titanate, for instance. Element 31 could be any source of radiant energy such as infrared rays, ultraviolet or other forms of radiation. The showing of element 31 is not intended to be a limitation on the type of radiant energy source utilized for initiating conduction.

In the process of growing crystals of a given material utilizing the apparatus of FIG. 1, the first step is the step of preparing a given material which is conductive at a temperature below its melting point so that an area thereof is in molten phase and the remainder of the given material is in substantially solid phase. Included in the step of preparing a given material is the step of selecting a seed of a given material of the highest Purity possible which may be either a conductor, such as the elemental metals, normally non-conductive material, such as silicon which is non-conductive at room temperature and certain dielectrics such as barium titanate, which become conductive on heating or semi-conductive materials, such as germanium, which are normally conductive at room temperature.

Also included in the step of preparing a given material is the step of either maintaining a gaseous atmosphere or maintaining a vacuum around the seed of the given material which has an area thereof in molten phase. Thus, depending on the type of given material whether it be conductive, normally non-conductive, or semi-conductive, one may be required to maintain an inert, oxidizing or reducing atmosphere or mixtures of these gases, or may be required to maintain a vacuum in the region of the seed if this is a condition for growing a crystal of that particular type of material. The step of preparing a seed of a given material which is normally non-conductive at room temperature includes the step of pre-heating the seed such as by irradiation with infrared, light, X- rays or radiant heat and should be applied in connection with materials such as silicon or barium titanate.

The final two-steps included in the step of preparing a given material in the process of growing crystals are the steps of positioning a radio frequency coil in juxtaposition to an area of the given material which is to be placed in a molten phase and the step of applying radio frequency energy by means of said coil to that area to melt that area of the given material.

The necessity that the material to be grown be conductive, or be capable of being made conductive can now be seen. In order to couple electromagnetic energy to a material, the material must be in an electrically conducting state. Since many materials which are normally non-conductive or, which are used as dielectric become conductive upon heating, the process of the present invention may be utilized with any material which can be made conductive at a temperature below its melting point.

The first main step in the process of growing crystals in accordance with the teachings of this invention is the step of subjecting the seed of given material to high frequency induction heating to render one portion of seed 1 molten while the remainder of the seed is in solid state. Thus, the application of radio frequency energy to the materials which are conductive, normally non-conductive or semiconductive causes an area of one of these materials to be raised to the melting point and by maintaining the application of radio frequency energy, as from coils 11, 12 of FIGS. 1 and 2, sufiicient power is coupled by induction heating to maintain the area of the given material in molten phase. The molten area is generally disposed at the tip of the seed of the given material and appears as shown in FIG. 2. In the course of experiments in perfactmg the steps of this process, some difficulty was experienced in coupling the proper amount of power to the given material and maintaining a given area thereof in molten condition by the application of radio frequency power. In some instances, it was found that a suflicient amount of power could not be coupled to the given material to raise the temperature to the melting point and in other cases it was found that too large an area was melted with consequent spill-over of the molten material, a circumstance which was deleterious to the completion of this process. It was found, however, that by proper design of the radio frequency coil and by the employment of field concentrators that, for any given material of a given size, sufiicient power could be coupled to the material and an optimum sized molten area could be maintained without difficulty. The annealing coil 11 of FIGS. 1 and 2 was found to aid in the production of improved crystals in that coil 11 maintained a sufficiently high temperature to prevent sharp thermal gradients in the crystal being grown, thereby preventing cracking of the crystal.

The second basic step in the process of growing crystals in accordance with the teachings of this invention is the step of dissolving additional material in said molten porton of said seed.

Included in the step of dissolving additional material in the molten portion of said seed is the step of introducing solid particles having the same composition as that of the given material. Thus, if it is desired to grow a crystal of a material which is conductive, normally nonconductive or semi-conductive, solid particles having the same composition as that of the given material would be introduced into the molten area at the tip of the seed. For instance, if it is desired to grow a crystal of germanium, germanium in solid particle form would be introduced into the molten area of the germanium. Also, included in the step of adding a given quantity of a desired material to dissolve in the molten area is the step of introducing solid particles having the same composition as that of the given material and simultaneously introducing solid particles of other compositions which act to change the properties of the given material. By utilizing the foregoing step, therefore, it is possible to introduce into semiconductors such as germanium, for instance, well-known donor or acceptor impurities such as aluminum or arsenic in solid particle form. In this manner, then, the conductivity type of the semi-conductive material may be changed without interrupting the process. Utilizing this technique of simultaneously introducing solid particles of other composition, the magnetic properties, the dielectric constant or the tensile and compressive properties of the given material may likewise be varied at will. A step which produces the same result as the foregoing step, which is included in the step of dissolving additional material in the molten portion of the seed is the step of introducing solid particles having the same composition as that of the given material and simultaneously introducing a vapor of another composition which acts to change the properties of the given material. In this connection, for

instance, silicon may have impurities introduced into it by introducing the gases arsine, AsH or pentaborane-9, B H Other doping agents which may be utilized to vary the conductivity type of silicon are stibine, S H which produces a conductivity of the N-type and halides of boron which produce conductivity of the P-type. The introduction of doping agents in vapor form into the region of growth or atmospheric doping as it may be termed, of course, may only be utilized with doping agents or compounds which on the dissociation do not result in products which contaminate the resulting crystal.

The final basic step, according to the teachings of this invention is the step of withdrawing the seed from the zone of induction heating to control the growth of the solid portion thereof. That is, to permit solidification in crystalline form of the molten area as the desired material accumulates in liquid form in the molten area. In this step, the seed of the given material is slowly withdrawn as the material being added accumulates in liquid form in the molten area. The rate of withdrawal is adjusted so that cooling is not accomplished too rapidly and so that a portion or an area at one end of the seed of the given material is always maintained in molten condition. This step includes the step of rotating the seed to prevent undesirable build up of material as the particles accumulate in the molten portion of the seed.

The steps in the above process have been recited in the broadest terms possible with only a few examples included to indicate to the art the almost universal applicability of this process in the growth of crystals which fall into the categories of electrically conductive materials, normally non-conducting materials and semi-conductive materials.

In the paragraphs which follow examples of the various constituents used, examples of the various parameters involved and the steps in the growth of various crystal types will be discussed.

As has been mentioned previously, the given material which is conductive below its melting point may be a metal. The metals preferably utilized in this process are those which are normally solid at room temperature and which are conductive at room temperature. Thus,-metals such as hafnium, beryllium, titanium, vanadium, chromium, zirconium, tungsten or molybdenum may be utilized and either single crystals or poly-crystalline forms of these metals may be grown by the process of this invention.

In this process, materials which are normally non-conductive but which can be rendered electrically conducting upon heating may also be utilized. Materials such as silicon, a semiconductor, and barium titanate, a dielectric, fall into the category of substances which are normally nonconductive but which become conductive by elevating their temperature from room temperature to some temperature below their melting point. The change in temperature, in many instances, is very slight as in the case of Silicon when this material is rendered conductive by the application of light. As an alternative, a resistive element 31 as shown in FIG. 1, may be placed around a seed of the normally non-conductive material and heated to a high temperature by radio frequency induction heating. The radiation from the ring in the form of heat then raises the temperature of the material to a point'where conduction is initiated. After conduction has been initiated, the ring may be re moved and heating is continued by radio frequency means alone. By the utilization of the technique of the present invention, crystals of dielectrics, such as strontium and barium titanate, may be grown.

With reference to the type of semiconductors which may be utilized in the practice of this invention, it has been found that the well-known semiconductors, germanium and silicon, can be grown in single crystal or polycrystalline form with little difficulty. Other semiconductive materials, such as ferrites, which have the general formula where O is oxygen, Fe is iron and X may be a divalent metal such as zinc, nickel, or cobalt, or two metals, such as nickel-zinc, manganese-magnesium, or cobalt-copper, or more than two of these metals taken together, can also be grown in crystalline form utilizing the technique of this invention. The fact that ferrites can be grown in crystalline form is particularly significant in that more and more applications are being found which utilize the particular characteristics inherent in these materials. In connection with the growth of ferrite crystals, it should be noted that characteristics such as permeability and dielectric constant and saturation magnetization may be varied during growth by simply varying the composition of the particles being introduced. Further, it has been found that the ferrites need not have exactly the same crystal lattice structure and for this reason materials having a similar lattice structure may be introduced, thereby producing a crystal which is composed of, in one portion, a cobalt ferrite and, in another portion, a nickel ferrite. The process of the present invention may also be utilized in growing alloys which have special and desirable characteristics and which, at the present time, can only be obtained with great difficulty. Materials which fall into this category are high temperature alloys of titanium and cobalt.

Other applications, such as the direct growth of atomic reactor fuel elements, are entirely possible using the technique of this invention. Another possible application is the production of high temperature turbine blades which are presently manufactured by the expensive and wasteful technique of skull melting.

In connection with the various parameters which must be varied in the practice of this invention, it has been found that the temperature in the region of the molten area at one end of the seed of the given material should be preferably at the melting point. It is possible, however, to utilize this technique even at temperatures which are higher than the melting point. The temperature may be changed in an upward direction depending on rate of growth on whether one desires to grow a single or polycrystalline form, or depending on which crystal form is desired of a material which has several crystal forms. The size of the particles utilized in this process is not critical. The only criterion which should be fulfilled is that the particles be small enough to dissolve without difficulty in the molten area at the tip of the seed. Experimental results, however, have indicated that the particle sizes should be smaller when it is desired to grow a single crystal than when growing a crystal which has a polycrystalline form. It has been found that there is no theoretical limitation as to the amount of pressure of the surrounding atmosphere which may be used when carrying out the process of this invention. Some materials are grown more conveniently in a vacuum and others are grown more conveniently under pressures of several atmospheres. In instances, such as when atmospheric doping is being utilized, it may be convenient to increase the pressure of the doping vapor in order to suppress vaporization of the material that is being doped. The size of crystals which can be grown utilizing the technique of the present invention is governed by the amount of power available and the shape of the resulting crystal is governed by the shape of the radio frequency field obtained. The length of the crystal which can be grown is theoretically unlimited because the crystal can be grown in a continuous manner. The materials utilized in the growth of the crystal can be fed continuously from a source which is easily replenished and which does not require the cessation of growing to recharge with new materials in particle form. Thus, by maintaining a molten area at the tip of the crystal fixed relative to the radio frequency coil, it is impossible to grow crystals continuously. From this, it may be seen that the rate of withdrawal of the crystal is governed by the rate of accumulation of the melted particles in the molten area at the tip of the crystal. The practically universal application of this process is further indicated by the fact that crystals, if a step in their growth requires it, may

be grown in vacuum, in inert atmospheres, in oxidizing atmospheres or in reducing atmospheres or in mixed gaseous atmospheres. The only criterion to be utilized in connection with the provision of gaseous atmospheres in crystal growth is that any gaseous atmosphere may be introduced as long as the gases do not react with each other and do not contaminate the crystal being grown.

Up until this point, the process of growing crystals utilizing the method of this invention has been discussed in rather general terms. In the following paragraphs, examples of a conductive material, a normally nonconductive material and several semiconductive materials will be described in detail. The steps of the processes in connection with these various materials are outlined hereinbelow.

The steps in the process of growing a crystal of a ferrite material are as follows:

(1) Selecting a seed of a given ferrite material having the highest purity possible. Ferrites having the general formula XOFe O for instance, may be used.

(2) Preparing a given material of the same composition as the given ferrite material in particle size.

(3) Positioning the seed of ferrite material in an oxygen-nitrogen atmosphere. The oxygen-nitrogen atmosphere is utilized to maintain the chemical balance of the ferrite, that is, to make certain that on melting the ferrite does not release oxygen thereby disturbing the desired composition of the material.

(4) Applying radio frequency energy to the seed at one end so that the temperature at an area at one end of the seed is raised to the melting point.

(5) Adding the given ferrite material in particle size to the molten area while maintaining the application of radio frequency energy to maintain the area in a molten condition.

(6) Adjusting the particle feed-rate and power input to set the desired size of the crystal and rate of growth.

(7) Withdrawing the ferrite seed at a given rate to permit solidification in crystalline form of the molten area as the particles accumulate in liquid form in the molten area.

(8) Annealing by induction heating to prevent cracking of the crystal because of sharp thermal gradients.

In the above recited process the particle size of the ferrite material may be, for instance, less than mesh. At this point, it should be noted that the process utilizes no crucible and because of this, -a source of contamination not easily dealt with in some prior art schemes, has been eliminated. It should also be noted that the process of the present invention requires no combustion supporting atmosphere. That is, it is a fiameless process in which solid or gaseous particles melt or dissolve in the molten area at the tip of the seed. This step may be differentiated from the prior art step wherein solid particles of a given material are actually melted in a flame and are then allowed to accumulate in molten form at the tip of a seed. An advantage which follows directly from the elimination of a combustion supporting atmosphere is that a controllable atmosphere may now be utilized and further the introduction of impurities to vary the properties of the seed may be accomplished by using a gas or solid particles having a different composition from that of the given material.

The steps in the process of growing a crystal of silicon inchides the steps of irradiating a seed of silicon to initiate conduction and, further, shows the step of adding impurities to the silicon crystal by introducing impurities in vapor form. The steps in growing a crystal of silicon are as follows:

(1) Selecting a seed of the highest purity silicon.

(2) Preparing a material of the same composition as that of the silicon in particle size.

(3) Positioning the seed in a noble gas atmosphere such as helium, argon, neon, etc.

(4) Irradiating the seed to initiate conduction. Radiation in the form of infrared rays, gamma rays or X-rays may be utilized to initiate conduction. Any form of radiation may be used and is not limited to the examples given.

Applying radio frequency energy to the seed to raise the temperature at one end of the seed to the melting point. In connection with this step, it should be remembered that once conduction has been initiated, it is possible to couple radio frequency energy to the seed and in this manner raise the temperature at the tip of the seed to the melting point.

(6) Adding silicon in given particle size to dissolve in the molten area while maintaining the application of radio frequency energy to maintain the area at the end of the seed in a molten condition.

(7) Adjusting particle feed rate and power input to set desired size of crystal and rate of growth.

(8) Introducing at a desired point in growth, a vapor selected from the group consisting of arsine, stibine, pentaborane-9 aluminum trichloride and halides of boron to change the conductivity type of the silicon. In connection with the foregoing step, it should be pointed out that the amount of P-type or N-type impurity introduced can be very carefully controlled and that the number of parts per million of impurity introduced can be varied as the crystal growth progresses. Itis also possible to grow a crystal having N-type impurities and then change over to a crystan having P-type impurities while growing the crystal in a continuous manner.

(9) Withdrawing the seed at a rate to premit solidification in crystalline form of the molten area.

Annealing by inducting heating to prevent cracking of the crystal.

The steps of growing a crystal of germanium are shown below. This process is very similar to that of growing a crystal of silicon with the exception that irradiation is not required to initiate conduction. In this process, the step of introducing impurities to vary the conductivity of the germanium is shown byintroducing impurities in solid particle form rather than in vapor form as was done with silicon. v

(1) Selecting a seed of'germanium of the best purity material available.

(2)-Preparing a material of the same composition as the germanium in particle size.

(3) Preparing a doping material of a desired composition selected from the group consisting of aluminum, gallium, indium, phosphorous, arsenic and antimony in particle size.

(4) Positioning the seed of germanium in a noble gas atmosphere.

(5) Applying radio frequency energy to the seed at one end so that the temperature at an area at one end of the seed is raised to the melting point.

(6) Adding germanium in particle size to dissolve in the molten area while maintaining the application of radio frequency energy to maintain the area in a molten condition.

(7) Adjusting particle feed rate and power input to set desired size of crystals and rate of growth.

(8) Introducing at a desired point in growth one of the above-mentioned doping materials to change the conductivity of said germanium.

(9) Withdrawing the seed at a given rate to permit solidification in crystalline form of the molten area as the particles of germanium and doping material accumulate in liquid form in the molten area.

(10) Annealing by induction heating to prevent cracking of the crystal.

The steps in the growth of a crystal of hafnium which is a metal, normally conductive at room temperature are given below. This metal has been selected rather than one of the better known metals, such as cobalt or iron, to show the versatility of the process of this invention. Hafnium usually presents a number of difficulties in manufacture and handling, but ,as will be seen from a consideration of the steps in the growth of a crystal of this material, it may be handled with little difiiculty utilizing the teaching of this invention. I

(1) Selecting a seed of hafnium of the highest purity possible.

(2) Preparing a material of the same composition as said hafnium material in particle size.

(3) Positioning the seed in a vacuum.

(4) Applying radio frequency energy to the seed at one end thereof so that the temperature at anarea on the end of the seed is raised to the melting point.

(5) Adding the given hafnium material in particle size to the molten area to dissolve therein, while maintaining the application of radio frequency energy to maintain the area in a molten condition.

(6) Adjusting particle feed rate and power input to set desired size of crystal and rate of growth. 7

(7) Withdrawing the hafnium seed at a given rate to permit solidification in crystalline form of the molten area as said particles accumulate in liquid form in the molten area.

(8) Annealing by induction heating to prevent cracking of the crystal.

From the foregoing process, it may be seen that, if a material is most easily handled in a vacuum, the steps in this process provide for this contingency. Thus, hafnium is most easily handled in a vacuum but other metals may require a different type of atmosphere and instead of the step of positioning the seed of metal in a vacuum, the step of positioning the seed may be carried out in any given atmosphere depending upon the requirements for the particular metal utilized.

The steps of the process of this invention are shown adapted to a material which is a dielectric and which falls into the general category of being normally a non-conductor. The following process may be utilizedwith other dielectrics, such as strontium titanate and quartz without departing from the spirit of this invention. The steps for growing a crystal of barium titanate are as follows:

(1) Selecting a seed of barium titanate of'the highest purity available. V

(2) Preparing a material of the same composition as the barium titanate in particle size.

(3) Positioning the seed in an oxygen atmosphere at a desired pressure.

(4) Irradiating by means ofinfrared radiation the barium titanate seed to initiate conduction.

(5) Applying radio frequency energy to the seed at one end so that the temperature at an area on one end of the seed is raised to the melting point.

(6) Adding barium titanate in given particle size to the molten area to dissolve therein while maintatining the application of radio frequency energy to maintain the area in a molten condition.

(7) Adjusting particle feed rate and power input to set the desired size of crystal and rate of growth.

(8) Withdrawing the barium titanate seed at a given rate to permit solidification in crystalline form of said molten area as the particles accumulate in liquid form in the area.

(9) Annealing the crystal by induction heating to prevent cracking of said crystal.

While we have described the above principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. Apparatus for growing crystals from a given material which is conductive at a temperature below its melting point and which upon solidification from a melted phase is capable of forming a crystalline structure comprising:

an atmosphere controlled enclosure;

a movable pedestal disposed to extend into said enclosure along the longitudinal axis thereof;

a crystal seed of said material secured to the end of said pedestal within said enclosure;

a first high frequency induction heating coil having a truncated conical configuration disposed in said enclosure coaxial of said longitudinal axis having the smaller diameter thereof above and in juxtaposition to the tip of said crystal seed;

a field concentrator having a truncated conical configuration disposed in said enclosure concentric of and substantially coextensive with said first coil having only the smaller diameter end thereof in electrical contact with the smallest diameter turn of said first coil to concentrate the high frequency field of said first coil in the tip of said crystal seed to melt the tip of said crystal seed and leave the remainder of said crystal seed in a solid state;

means to selectively introduce additional material into the molten tip of said crystal seed;

means coupled to said pedestal to Withdraw said crystal seed from the smaller diameter of said first coil to produce growth of the solid portion of said crystal seed as said additional material is introduced; and

a second high frequency induction heating coil having a substantially cylindrical configuration disposed in said enclosure coaxial of said longitudinal axis coiled about and spaced from said crystal seed below the molten tip of said crystal seed in an electromagnetic coupling relation with said crystal seed to anneal the solid portion of said seed as said seed is withdrawn from the smaller diameter of said first coil.

2. Apparatus according to claiml further including means disposed in said enclosure to irradiate said crystal seed to render said material which is normally nonconductive at room temperature conductive at a temperature below its melting point;

3. Apparatus according to claim 1, wherein said means to introduce additional material includes means to introduce particles of said material to the molten tip of said crystal seed.

4. Apparatus according to claim 1, wherein said means to introduce additional material includes:

means to introduceparticles of said material to the molten tip of said crystal seed; and

means to introduce particles of a given material diflerent than said material to the molten tip of said crystal seed.

5. Apparatus according to claim 1, wherein said means to introduce additional material includes:

means to introduce particles of said material to the molten tip of said crystal seed; and

means to introduce a vapor of a given material different than said material to the molten tip of said crystal seed.

6. Apparatus according to claim 1, wherein said means to withdraw includes means to rotate said crystal seed as said crystal seed is withdrawn.

7. The process of growing crystals of a material normally nonconducting at room temperature which upon solidification from a molten phase is capable of forming a crystalline structure comprising the steps of:

irradiating a crystal seed of said material to render said material conducting at a temperature below its melting point;

passing a first induction heating field through said irradiated crystal seed to melt the tip thereof and leave the remainder of said crystal seed in a solid state;

introducing additional material to the molten tip of said crystal seed;

withdrawing said crystal seed from said first heating field as said additional material is introduced to control the growth of the solid portion of said crystal seed; and

passing a second induction heating field through the solid portion of said crystal seed to anneal the solid portion of said crystal seed as said crystal seed is withdrawn from said first heating field.

References Cited in the file of this patent UNITED STATES PATENTS 2,686,864 Wroughton Aug. 17, 1954 2,686,865 Kelly Aug. 17, 1954 2,793,103 Emeis May 21, 1957 2,870,309 Capita Jan. 20, 1959 2,893,847 Schweickert July 7, 1959 2,897,329 Matare- July 28, 1959 FOREIGN PATENTS 774,270 Great Britain May 8, 1957 901,413 Germany July 8, 1949 88,128 Holland Apr. 17, 1958 OTHER- REFERENCES Keck: Review of Scient. Instr., vol. 25, pages 298 and 299 (1954). 

1. APPARATUS FOR GROWING CRYSTALS FROM A GIVEN MATERIAL WHICH IS CONDUCTIVE AT A TEMPERATURE BELOW ITS MELTING POINT AND WHICH UPON SOLIDIFICATION FROM A MELTED PHASE IS CAPABLE OF FORMING A CRYSTALLINE STRUCTURE COMPRISING: AN ATMOSPHERE CONTROLLED ENCLOSURE; A MOVABLE PEDESTAL DISPOSED TO EXTEND INTO SAID ENCLOSURE ALONG THE LONGITUDINAL AXIS THEREOF; A CRYSTAL SEED OF SAID MATERIAL SECURED TO THE END OF SAID PEDESTAL WITHIN SAID ENCLOSURE; A FIRST HIGH FREQUENCY INDUCTION HEATING COIL HAVING A TRUNCATED CONICAL CONFIGURATION DISPOSED IN SAID ENCLOSURE COAXIAL OF SAID LONGITUDINAL AXIS HAVING THE SMALLER DIAMETER THEREOF ABOVE AND IN JUXTAPOSITION TO THE TIP OF SAID CRYSTAL SEED; A FIELD CONCENTRATOR HAVING A TRUNCATED CONICAL CONFIGURATION DISPOSED IN SAID ENCLOSURE CONCENTRIC OF AND SUBSTANTIALLY COEXTENSIVE WITH SAID FIRST COIL HAVING ONLY THE SMALLER DIAMETER END THEREOF IN ELECTRICAL CONTACT WITH THE SMALLEST DIAMETER TURN OF SAID FIRST COIL TO CONCENTRATE THE HIGH FREQUENCY FIELD OF SAID FIRST COIL IN THE TIP OF SAID CRYSTAL SEED TO MELT THE TIP OF SAID CRYSTAL SEED AND LEAVE THE REMAINDER OF SAID CRYSTAL SEED IN A SOLID STATE; 